1. Field of the Invention
The present invention relates to the field of laser processing methods and systems, and specifically to laser processing methods and systems for removing one or more conductive target link structures formed on substrates. This invention is particularly applicable, but not limited to, laser repair of redundant semiconductor memory devices.
2. Description of the Related Art
Economics and device performance have driven the size for the DRAMs and logic devices to very small physical dimensions. Not only are the devices small, but the interconnects and links thickness have also decreased dramatically in recent years.
Some thermal laser processing of links, for example, as described in “Link Cutting/Making” in HANDBOOK OF LASER MATERIALS PROCESSING, Chapter 19, pp. 595-615, Laser Institute of America (2001), relies on the differential thermal expansion between the oxide above the link and the link itself. The differential expansion results in a high pressure build-up of the molten link contained by the oxide. The oxide over the link is necessary to contain the link in a molten state long enough to build-up sufficient pressure to crack the oxide and explosively expel the link material. If the pressure is too low, the link will not be removed cleanly. Alternative laser wavelengths and laser control strive to increase the laser “energy window” without damaging the substrate and material contiguous to the link.
Further information is available regarding link blowing methods and systems, including material processing, system design, and device design considerations, in the following representative U.S. patents and published U.S. applications: U.S. Pat. Nos. 4,399,345; 4,532,402; 4,826,785; 4,935,801; 5,059,764; 5,208,437; 5,265,114; 5,473,624; 6,057,180; 6,172,325; 6,191,486; 6,239,406; 2002-0003130; and 2002-0005396.
Other representative publications providing background on link processing of memory circuits or similar laser processing applications include: “Laser Adjustment of Linear Monolithic Circuits,” Litwin and Smart, ICAELO, (1983); “Computer Simulation of Target Link Explosion In Laser Programmable Memory,” Scarfone, Chlipala (1986); “Precision Laser Micromachining,” Boogard, SPIE Vol. 611 (1986); “Laser Processing for Application Specific Integrated Circuits (ASICS),” SPIE Vol. 774, Smart (1987); “Xenon Laser Repairs Liquid Crystal Displays,” Waters, Laser and Optronics, (1988); “Laser Beam Processing and Wafer Scale Integration,” Cohen (1988); “Optimization of Memory Redundancy Link Processing,” Sun, Harris, Swenson, Hutchens, Vol. SPIE 2636, (1995); “Analysis of Laser Metal Cut Energy Process Window,” Bernstein, Lee, Yang, Dahmas, IEEE Trans. On Semicond. Manufact., Vol. 13, No. 2. (2000); “Link Cutting/Making” in HANDBOOK OF LASER MATERIALS PROCESSING, Chapter 19, pp. 595-615, Laser Institute of America (2001).
Requirements for the next generation of dynamic random access memory (DRAM) include fine pitch links having link widths less than 0.5 microns and link pitch (center to center spacing) less than 2 microns (e.g., 1.33 microns). Current commercial laser memory link repair systems, which use q-switched, Nd based solid-state lasers with wavelengths of about 1 to 1.3 microns and pulse widths about 4 to 50 nanoseconds (ns), are not well suited for meeting such requirements. The large (wavelength limited) spot size and thermal effect (pulse width limited) are two limiting factors.
In INTERNATIONAL JOURNAL OF ADVANCED MANUFACTURING TECHNOLOGY (2001) 18:323-331, results of copper laser processing are disclosed. A frequency tripled Nd:YAG laser with 50 nanosecond (ns) pulse duration was used. The measured heat affected zones (HAZ) were about 1 micron for 6×108 W/cm2 irradiance and more than 3 microns for about 2.5×109 W/cm2 irradiance.
Attempts have been made to address the problems. Reference is made to the following U.S. patents and published applications: U.S. Pat. Nos. 5,208,437; 5,656,186; 5,998,759; 6,057,180, 6,300,590; 6,574,250; WO 03/052890; and European patent EP 0902474. In summary, the conventional q-switched, nanosecond solid-state lasers, even at short wavelengths, are not able to process the fine pitch links due to its thermal process nature. Material interaction may be a substantially non-thermal process at femtosecond pulse widths, but the complexity, high costs, and reliability of femtosecond pulse lasers may limit practical implementations. Device and material modifications to support laser repair are expensive and alone may not be sufficient. An improved method and system for fine pitch link processing is needed to circumvent problems associated with thermal effects yet provide for efficient link removal at high repetition rates without the complexity associated with femtosecond laser systems.
The following references [1]-[12] are also related to the present invention, some of which are referenced herein:
[1] J. Lee, J. Ehrmann, D. Smart, J. Griffiths and J. Bernstein “Analyzing the process window for laser copper-link processing,” Solid State Technology, pp. 63-66, December 2002.
[2] J. B. Bernstein, J. Lee, G. Yang and T. Dahmas, “Analysis of laser metal-cut energy process window,” IEEE Semiconduct. Manufact., Vol. 13, No. 2, pp. 228-234, 2000.
[3] J. Lee and J. B. Bernstein, “Analysis of energy process window of laser metal pad cut link structure,” IEEE Semiconduct. Manufact., Vol. 16, No. 2, pp. 299-306, May 2003.
[4] J. Lee and J. Griffiths, “Analysis of laser metal cut energy process window and improvement of Cu link process by unique fast rise time laser pulse,” Proceedings of Semiconductor Manufacturing Technology Workshop, Hsinchu, Taiwan, pp. 171-174, December 2002.
[5] T. Kikkawa, “Quarter-micron interconnection technologies for 256M Drams,” Extended Abstracts, Int. Conf. Solid Devices and Materials, pp. 90-92, 1992.
[6] M. D. Perry, B. C. Stuart, P. S. Banks. M. D. Feit and J. A. Sefcik, “Ultrafast Laser for Materials Processing,” p. 82 and pp. 499-508, LIA Handbook of Laser Materials Processing, Laser Institute of America, Magnolia Publishing, Inc., 2001.
[7] H. Liu, G. Mourou, Y. N. Picard, S. M. Yalisove and T. Juhasz, “Effects of Wavelength and Doping Concentration on Silicon Damage Threshold,” Laser and Electro-Optics, Vol. 2, p. 2, May 2004.
[8] G. Pasmanik, “Pico versus Femto in Micromachining,” Optoelectronics World, pp. 221-224, June 2001.
[9] J. Jandeleit, G. Urbasch, H. D. Hoffmann, H. G. Treusch and E. W. Kreutz, “Picosecond Laser Ablation of Thin Copper Films,” Appl. Phys., Vol. A 63, pp. 117-121, 1996.
[10] J. C. North and W. W. Weick, “Laser Coding of Bipolar Read-Only Memories,” IEEE Journal of Solid State Circuits, Vol. SC-11, No. 4, pp. 500-505, 1976.
[11] J. B. Bernstein, S. S. Cohen and P. W. Wyatt, “Metal Wire Cutting by Repeated Application of Low-Power Laser Pulses,” Rev. Sci. Instrum., 63(6), pp. 3516-3518, 1992.
[12] M. Lapczyna, K. P. Chen, P. R. Herman, H. W. Tan and R. S. Marjoribanks, “Ultra high repetition rate (133 MHz) laser ablation of aluminum with 1.2-ps pulses,” Appl. Phys., Vol. A 69 [Suppl.], S883-S886, 1999.